This invention relates to the art of energy conversion, and more particularly to a new and improved method and apparatus for generating steam from the combustion of fuel which steam can be used for example in providing heat or mechanical energy.
The combustion of fuel to produce steam for such uses as the generation of electricity, space heating, and industrial process steam accounts for a substantial part of the energy consumption of the United States. An inherent limitation of fuel-fired steam boilers is that the rate of heat transfer to the water and steam in the boiler is somewhat less than the rate at which chemical energy is released in the combustion of the fuel. Since no other source of heat is utilized, the heat transfer to the water and steam cannot be greater than the release of chemical energy, and is generally somewhat less due to energy losses to the environment, particularly the loss of the enthalpy of the hot flue gases.
The combustion of fuel to obtain mechanical energy by means of internal combustion engines accounts for another substantial part of the energy consumption of the United States. At best, the mechanical power output of a conventional internal combustion engine is only about thirty-five percent of the rate at which chemical energy is released in the combustion of the fuel. The remainder of the energy is lost to the environment, primarily by loss of the enthalpy of the hot exhaust gases and loss of heat through surfaces adjacent to the combustion chamber. The term waste heat is commonly used to refer to the sum of the heat passing through surfaces adjacent to the combustion chamber and the heat which could be obtained from the hot exhaust gases as they cool to the temperature of the environment. In a typical piston engine, the waste heat output is divided fairly evenly between the exhaust gases and the heat lost through surfaces such as piston crown and cylinder wall which are adjacent to the combustion chamber, while in other engines such as gas turbines, the exhaust gases may represent nearly all of the waste heat output. In some piston engines and other engines, much of the heat passing through surfaces adjacent to the combustion chamber is not transmitted directly to the environment, but is transmitted to an intermediate coolant fluid, such as water or oil, which in turn transmits heat to the environment.
The fundamental problem in converting the waste heat ouput of an internal combustion engine to useful work is the relatively low temperature at which the heat is available. For example, in the case of heat transmitted to the environment by a coolant, the coolant temperature is typically in the neighborhood of 100.degree.C. An auxiliary heat engine using the internal combustion engine coolant as a heat source could convert only a small fraction of the heat to mechanical energy; thus, operation of such an auxiliary heat engine is usually not practical. The exhaust gases from an internal combustion engine are at a higher temperature, typically in the neighborhood of 450.degree.C; but, if all of the available heat is to be removed from the exhaust gases, the heat will be obtained over a continuous range of temperature from 450.degree.C to the temperature of the environment. In general, it is practical to use only the heat obtained from the upper part of this temperature range. Such is the case in typical combined gas turbine-steam turbine cycles where an electrical generator is driven by a gas turbine and where heat from the gas turbine exhaust is used to produce steam for the generation of additional electricity. Very little heat is recovered from the exhaust gases below a temperature of about 200.degree.C.
In order to convert as much as thirty-five percent of the chemical energy of its fuel to mechanical energy, an internal combustion engine must be designed for efficient operation and then must be operated at optimum conditions. For example, with any engine there is a combination of load and speed at which the engine operates most efficiently with respect to fuel utilization. In some applications of internal combustion engines, the engine can be operated near its optimum efficiency most of the time. In other applications, especially in the field of transportation, operation conditions such as load and speed may vary considerably and may often be far from optimum, with the result that the mechanical energy output falls to a value considerably below thirty-five percent of the chemical energy of the fuel, while the waste heat output increases to a value considerably above sixty-five percent. Automobiles have particularly poor efficiency with respect to fuel utilization. A typical automobile engine achieves its best efficiency when operating near full power, but full power operation is rarely required under typical driving conditions. Most of the time, only a small fraction of the maximum engine power is used. Some improvement in efficiency can be obtained, at the expense of maximum rate of acceleration, by putting smaller engines in automobiles so that a greater fraction of the maximum engine power is being used at any given time. Nevertheless, so long as the kinetic and potential energies of a vehicle vary under typical driving conditions, it is not practical to operate the engine continuously at full power unless the vehicle is equipped with a fairly efficient energy storage means such as a mechanical flywheel or other energy reservoir. If the vehicle is so equipped, the engine can be operated at full power to add energy to the reservoir and can be shut off whenever the energy level in the reservoir is adequate.